Calcium hydroxyapatite (HA) plasma sprayed coatings are widely used for implantable device coatings. Basically, such a process entails coating powder particles passed through a plasma flame where surface melting (at least) occurs, enabling the particles to stick to a device surface and to each other. However, HA is difficult to melt; indeed, it has no melt phase and melts only with decomposition. Controlling the coating process to provide sufficient melting for coating adhesion and formation without excessive decomposition of the HA powder is difficult. Further, mechanical forces and abrasion during surgical implantation can damage an HA coating.
After implantation, such a coating helps to accelerate bone formation around the implant device. In difficult sites, such as where there is mechanical movement or because of poor implant stability, bone integration may not occur: The result can be fibrous tissue encapsulation. Bone formation may also not be successful in patients with certain medical problems that inhibit healing such as diabetes, or in patients who are smokers, have poor nutrition, or have bone that has been compromised by radiation treatments, for example. While standard coatings do well in most cases, they can be inadequate in such marginal applications, and a more bioactive coating would be advantageous.
As summarized, such coatings have several well-recognized deficiencies. Problems relate to production, substrate bonding strength, bonding strength within the coating, and the speed of tissue integration. As a result, there remains an on-going search in the art for improved calcium phosphate coatings and particulate precursors.